So you would prefer the version of a parallel world:
"After grabbing the chest i auto- ran towards the boss and took a sip or two of my tea.
After I placed down the cup to start the fight, i witnessed the Boss doing the final slap to kill the DNC."
Same outcome.

Also, pulling the boss when not everyone is at the arena gives the boss a advantage.
Because they might not be prepared for mechanic and run into their death because they try to get in the arena on time.

So I could also report the DNC for doing that.

To repeat myself for a last time.
It was a absurd and funny situation to see a dancer dashing and sprinting alone towards the boss, so i let him tank the boss for a while.

If you don't like that, then feel free to report any person that is doing this towards you.
When you have to report a lot of people for this, then it might be your gameplay that brings you in situation like that.